1. Field of the Invention
The invention relates to a method for removing debris from the bottom of a large water-filled vessel having internal mechanisms which obstruct the bottom of the vessel, and more particularly to a method for removing debris from the bottom of a nuclear reactor vessel without removing the entire core assembly or reactor components located below the core assembly.
2. Description of the Prior Art
In the course of operating and maintaining a nuclear reactor, such as a pressurized water reactor, it is possible for debris to build up in the cooling water circulating through the reactor vessel. Such debris has various sources. For example, normal wear within the vessel can generate bits of metal or broken parts which end up circulating with the cooling water. Further, a ruptured fuel tube could eject pieces of fuel pellets into the cooling water. Debris can also come from sources external to the reactor vessel. For example, metal particles generated during a welding repair to a steam generator connected to the closed loop cooling system of the reactor could get into the cooling water and circulate into the reactor vessel.
In a pressurized water reactor, cooling water enters the vessel through a nozzle located in the upper portion of the side wall of the vessel. An annulus created by the inside wall of the vessel and the core barrel provides a path for the incoming cooling water to flow down along the inside perimeter of the vessel wall toward the bottom of the vessel where it rises up through the central portion of the vessel, passing through the core assembly and other control apparatus within the vessel commonly referred to as reactor internals and exiting through an outlet nozzle positioned in the upper portion of the vessel side wall. Debris at the bottom of the vessel or otherwise circulating with the cooling water is thereby carried through the internals of the vessel where it could become lodged between parts. For example, particles of debris could get lodged between the fuel rods, and over a period of time cause extensive wear, resulting in a rupture of one or more fuel rods. Also, particles could get lodged between a control rod and its housing, causing failure of the control rod. Another area for potential problems due to debris in the cooling water is the drive lines which are connected to the control rods. The drive lines are used for moving the control rods in and out of the core assembly. There are spaces in the area of the drive lines as small as 6.25.times.10.sup.-4 mm in which particles of debris could become lodged and cause mechanical problems.
It is known to use a filtration system to filter the cooling water during a refueling outage. This is done to improve the clarity of the water so that the internals of the reactor vessel can be more clearly seen through the water when performing the various tasks involved in the refueling operation. One such filtration system comprises a pump, motor and filter assembly which operates to take water from the reactor cavity, circulate it through the filter assembly to remove particles larger than the filter pore size, and return the clarified water to the reactor cavity. The intake for this known filtration system, however, is located near the top of the refueling cavity and thus does not operate to remove any debris which has settled to the bottom of the reactor vessel which is located below the refueling cavity floor. Accordingly, while the clarity of the water is improved for the refueling operation, once the reactor is put back into operation and the cooling water begins to circulate, the debris which has settled to the bottom of the vessel may circulate up through the reactor internals, presenting problems of the type previously discussed.
It is known from U.S. Pat. Nos. 3,809,608 and 4,309,583 to withdraw water from the bottom of a reactor vessel by means of a pump connected to a pipe which extends to the bottom of the vessel. However, in each of the above noted U.S. Patents the pumping mechanisms for removing water from the vessel are employed in conjunction with a complex heat-treating apparatus which is inserted into the vessel after all of the internals have been removed from the vessel. The heat-treating apparatus includes a seal for effecting a water-tight seal between the inner confines of the vessel and the refueling cavity so that after the heat-treating apparatus is inserted into the vessel the pumping mechanism can empty all of the water out of the vessel into the refueling cavity in preparation for the heat treatment to be applied to the reactor vessel wall. Accordingly, neither of these two U.S. patents address the problem of removing debris from the bottom of the reactor vessel without completely removing the reactor internals and without withdrawing all of the water from the reactor vessel.
For the purposes of the following description and the claims, the term upper reactor internals refers to the upper core plate and the reactor internals located above the upper core plate.